Biosensors provide an analysis of a biological fluid, such as whole blood, serum, plasma, urine, saliva, interstitial, or intracellular fluid. Typically, biosensors have a measurement device that analyzes a sample residing in a sensor strip. The sample usually is in liquid form and in addition to being a biological fluid, may be the derivative of a biological fluid, such as an extract, a dilution, a filtrate, or a reconstituted precipitate. The analysis performed by the biosensor determines the presence and/or concentration of one or more analytes, such as alcohol, glucose, uric acid, lactate, cholesterol, bilirubin, free fatty acids, triglycerides, proteins, ketones, phenylalanine or enzymes, in the biological fluid. The analysis may be useful in the diagnosis and treatment of physiological abnormalities. For example, a diabetic individual may use a biosensor to determine the glucose level in whole blood for adjustments to diet and/or medication.
Biosensors may be designed to analyze one or more analytes and may use different sample volumes. Some biosensors may analyze a single drop of whole blood, such as from 0.25-15 microliters (μL) in volume. Biosensors may be implemented using bench-top, portable, and like measurement devices. Portable measurement devices may be hand-held and allow for the identification and/or quantification of one or more analytes in a sample. Examples of portable measurement devices include the Ascensia Breeze® and Elite® meters of Bayer HealthCare in Tarrytown, N.Y., while examples of bench-top measurement devices include the Electrochemical Workstation available from CH Instruments in Austin, Tex. Biosensors providing shorter analysis times, while supplying the desired accuracy and/or precision, provide a substantial benefit to the user.
Biosensors may use optical and/or electrochemical methods to analyze the sample. In some optical systems, the analyte concentration is determined by measuring light that has interacted with or been absorbed by a light-identifiable species, such as the analyte or a reaction or product formed from a chemical indicator reacting with the analyte. In other optical systems, a chemical indicator fluoresces or emits light in response to the analyte when illuminated by an excitation beam. The light may be converted into an electrical output signal, such as current or potential, which may be similarly processed to the output signal from an electrochemical method. In either optical system, the biosensor measures and correlates the light with the analyte concentration of the sample.
In electrochemical biosensors, the analyte concentration is determined from an electrical signal generated by an oxidation/reduction or redox reaction of the analyte or a species responsive to the analyte when an input signal is applied to the sample. The input signal may be applied as a single pulse or in multiple pulses, sequences, or cycles. An oxidoreductase, such as an enzyme or similar species, may be added to the sample to enhance the electron transfer from a first species to a second species during the redox reaction. The enzyme or similar species may react with a single analyte, thus providing specificity to a portion of the generated output signal. Examples of some specific oxidoreductases and corresponding analytes are given below in Table I.
TABLE IOxidoreductase (reagent layer)AnalyteGlucose dehydrogenaseβ-glucoseGlucose oxidaseβ-glucoseCholesterol esterase; cholesterol oxidaseCholesterolLipoprotein lipase; glycerol kinase; glycerol-3-Triglyceridesphosphate oxidaseLactate oxidase; lactate dehydrogenase;LactatediaphorasePyruvate oxidasePyruvateAlcohol oxidaseAlcoholBilirubin oxidaseBilirubinUricaseUric acidGlutathione reductaseNAD(P)HCarbon monoxide oxidoreductaseCarbon monoxide
A mediator may be used to maintain the oxidation state of the enzyme. Table II, below, provides some conventional combinations of enzymes and mediators for use with specific analytes.
TABLE IIAnalyteEnzymeMediatorGlucoseGlucose OxidaseFerricyanideGlucoseGlucose DehydrogenaseFerricyanideCholesterolCholesterol OxidaseFerricyanideLactateLactate OxidaseFerricyanideUric AcidUricaseFerricyanideAlcoholAlcohol OxidasePhenylenediamine
Electrochemical biosensors usually include a measurement device having electrical contacts that connect with electrical conductors in the sensor strip. The conductors may be made from conductive materials, such as solid metals, metal pastes, conductive carbon, conductive carbon pastes, conductive polymers, and the like. The electrical conductors typically connect to working, counter, reference, and/or other electrodes that extend into a sample reservoir. One or more electrical conductors also may extend into the sample reservoir to provide functionality not provided by the electrodes.
In many biosensors, the sensor strip may be adapted for use outside, inside, or partially inside a living organism. When used outside a living organism, a sample of the biological fluid is introduced into a sample reservoir in the sensor strip. The sensor strip may be placed in the measurement device before, after, or during the introduction of the sample for analysis. When inside or partially inside a living organism, the sensor strip may be continually immersed in the sample or the sample may be intermittently introduced to the strip. The sensor strip may include a reservoir that partially isolates a volume of the sample or be open to the sample. Similarly, the sample may continuously flow through the strip or be interrupted for analysis.
The measurement device applies an input signal through the electrical contacts to the electrical conductors of the sensor strip. The electrical conductors convey the input signal through the electrodes into the sample present in the sample reservoir. The redox reaction of the analyte generates an electrical output signal in response to the input signal. The electrical output signal from the strip may be a current (as generated by amperometry or voltammetry), a potential (as generated by potentiometry/galvanometry), or an accumulated charge (as generated by coulometry). The measurement device may have the processing capability to measure and correlate the output signal with the presence and/or concentration of one or more analytes in the biological fluid.
In conventional amperometry, current is measured during a read pulse as a constant potential (voltage) is applied across the working and counter electrodes of the sensor strip. The measured current is used to quantify the analyte in the sample. Amperometry measures the rate at which an electrochemically active, thus measurable, species is being oxidized or reduced at or near the working electrode. Many variations of the amperometric method for biosensors have been described, for example in U.S. Pat. Nos. 5,620,579; 5,653,863; 6,153,069; and 6,413,411.
A disadvantage of conventional amperometric methods is the non-steady-state nature of the current after a potential is applied. The rate of current change with respect to time is very fast initially and becomes slower as the analysis proceeds due to the changing nature of the underlying diffusion process. Until the consumption rate of the ionized measurable species at the electrode surface equals the diffusion rate, a steady-state current cannot be obtained. Thus, conventional amperometry methods that measure the current during the transient period before a steady-state condition is reached, may provide more inaccuracy than if the measurement is taken during a steady-state time period.
The measurement performance of a biosensor is defined in terms of accuracy and/or precision. Increases in accuracy and/or precision provide for an increase in measurement performance for the biosensor. Accuracy may be expressed in terms of bias of the biosensor's analyte reading in comparison to a reference analyte reading, with larger bias values representing less accuracy, while precision may be expressed in terms of the spread or variance among multiple analyte readings in relation to a mean. Bias is the difference between a value determined from the biosensor and the accepted reference value and may be expressed in terms of “absolute bias” or “relative bias”. Absolute bias may be expressed in the units of the measurement, such as mg/dL, while relative bias may be expressed as a percentage of the absolute bias value over the reference value. Reference values may be obtained with a reference instrument, such as the YSI 2300 STAT PLUS™ available from YSI Inc., Yellow Springs, Ohio.
Many biosensors include one or more methods to correct the error associated with an analysis. The concentration values obtained from an analysis with an error may be inaccurate. The ability to correct these inaccurate analyses may increase the accuracy of the concentration values obtained. An error correction system may compensate for one or more errors, such as sample hematocrit content, which is different from a reference sample. For example, conventional biosensors may be configured to report glucose concentrations presuming a 40% (v/v) hematocrit content for a whole blood sample, regardless of the actual hematocrit content of the sample. In these systems, any glucose measurement performed on a whole blood sample containing less or more than 40% hematocrit will include error or bias attributable to the “hematocrit effect”.
In conventional biosensor sensor strips for determining glucose concentrations, glucose may be oxidized by an enzyme, which then transfers the electron to a mediator. This reduced mediator then travels to the working electrode where it is electrochemically oxidized. The amount of mediator being oxidized may be correlated to the current flowing between the working and counter electrodes of the sensor strip. Quantitatively, the current measured at the working electrode is directly proportional to the diffusion coefficient of the mediator. The hematocrit effect interferes with this process because the red blood cells block the diffusion of the mediator to the working electrode. Subsequently, the hematocrit effect influences the amount of current measured at the working electrode without any connection to the amount of glucose in the sample.
Hematocrit bias refers to the difference between the reference glucose concentration obtained with a reference instrument and an experimental glucose reading obtained from a biosensor for samples containing differing hematocrit levels. The difference between the reference and values obtained from the biosensor results from the varying hematocrit levels between specific whole blood samples.
In addition to the hematocrit effect, measurement inaccuracies also may arise when the measurable species concentration does not correlate with the analyte concentration. For example, when a sensor system determines the concentration of a reduced mediator generated in response to the oxidation of an analyte, any reduced mediator not generated by oxidation of the analyte will lead to the sensor system indicating that more analyte is present in the sample than is correct due to mediator background. Thus, “mediator background” is the bias introduced into the measured analyte concentration attributable to measurable species not responsive to the underlying analyte concentration.
In an attempt to overcome one or more of these disadvantages, conventional biosensors have attempted multiple techniques, not only with regard to the mechanical design of the sensor strip and reagent selection, but also regarding the manner in which the measurement device applies the electric potential to the strip. For example, conventional methods of reducing the hematocrit effect for amperometric sensors include the use of filters, as disclosed in U.S. Pat. Nos. 5,708,247 and 5,951,836; reversing the polarity of the applied current, as disclosed in WO 2001/57510; and by methods that maximize the inherent resistance of the sample.
Multiple methods of applying the input signal to the strip, commonly referred to as pulse methods, sequences, or cycles, have been used to address inaccuracies in the determined analyte concentration. For example, in U.S. Pat. No. 4,897,162 the input signal includes a continuous application of rising and falling voltage potentials that are comingled to give a triangular-shaped wave. Furthermore, WO 2004/053476 and U.S. Pat. Docs. 2003/0178322 and 2003/0113933 describe input signals that include the continuous application of rising and falling voltage potentials that also change polarity.
Other conventional methods combine a specific electrode configuration with a input signal adapted to that configuration. For example, U.S. Pat. No. 5,942,102 combines the specific electrode configuration provided by a thin layer cell with a continuous pulse so that the reaction products from the counter electrode arrive at the working electrode. This combination is used to drive the reaction until the current change verses time becomes constant, thus reaching a true steady state condition for the mediator moving between the working and counter electrodes during the potential step. While each of these methods balances various advantages and disadvantages, none are ideal.
As may be seen from the above description, there is an ongoing need for improved biosensors, especially those that may provide an increasingly accurate determination of the analyte concentration in less time. The systems, devices, and methods of the present invention overcome at least one of the disadvantages associated with conventional systems.